XSS vulnerabilities are by far the most prevalent type of website vulnerability, according to a new report (PDF) issued Tuesday by White Hat Security, a firm that specializes in website security. They allow attackers to inject code and graphics into familiar websites, creating opportunities to install malware and phish account details even as a browser's address bar bears the name of Google.com, Facebook.com or some other website the user trusts.

In many cases, XSS flaws are self-replicating, as was the case with the so-called Samy worm, which knocked MySpace out of commission after it added more than a million users to the creator's friends page. The JS-Yamanner exploit that hit Yahoo mail in 2006 and last year's quick-moving attack targeting Google Orkut are two other examples of web worms gone wild.

Given how virulent these pests can be, you'd think Facebook would show a little more alacrity in squashing them. But that isn't the case here. According to the Reg reader who alerted us, he first told Facebook personnel about the vulnerability in August.

"I tried to alert Facebook as quickly as was possible - however I received no further correspondence to my communications," he wrote in an email. "It should be worth noting, that according to Bugtraq I'm not the first one to be perplexed with contacting Facebook regarding a security problem!"

Over the past week, an ugly worm dubbed Koobface has menaced plenty of Facebook users with spam and malware attacks without the benefit of a gaping hole. We shudder to think what might happen if miscreants combined such an exploit with a vulnerability like this.

The Facebook hole works with Internet Explorer, Google Chrome, Opera, Safari and installations of Firefox that don't use the NoScript plugin (you do use it, yes?). El Reg has asked Facebook representatives for comment and will be sure to update if we get it.

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Within three hours of the posting of this story, the XSS holes were closed. Representatives from Facebook have yet to respond.